Ultraviolet radiation (UVR) is implicated in the pathogenesis of various human skin diseases including cutaneous malignancies. The molecular mechanism which underlies this is poorly understood. UVR can activate various signal transduction pathways and to induce the expression of specific genes. This is believed to be transduced through UVR-induced reactive oxygen species (ROS). Protein tyrosine phosphorylation controlled by coordinate action of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) plays an important part in these signal transduction processes. While there is no convincing evidence to show that any PTK can be activated by ROS, PTPs have been shown to be regulated by a redox mechanism. In fact, with a low pKa (usually below 6.0), activate site cysteine of PTPs is an ideal regulatory site for ROS. In preliminary studies, we have found that UVB causes reversible inactivation of PTPs in vitro and in vivo. The inactivation can be prevented and reversed by specific reducing reagents. In vitro, activated PTPs appeared to be more sensitive to UVR, while in keratinocytes, UVB caused approximately 20% loss of PTPs activity in cell extracts. This study is intended to fully characterize UVR-induced inactivation of PTPs in vitro with a number purified PTPs and in vivo with human keratinocytes and epidermal cell lines. We will also identify the PTPs that are most sensitive to UVR and define their implication in UVR- induced signal transduction and pathological consequences. The major experimental approaches include in vitro enzymatic assays, fractionation of cell extracts on chromatographic columns, specific labeling of oxidized PTPs, and in-gel PTP assays. This study should help to open a new field related to skin diseases and it may lead to therapeutic implications.